Shear type piezo-electric device



Sept. 1, 1959 s. ROBERTS SHEAR TYPE PIEZO-ELECTRIC DEVICE Filed Oct. 50,1952 (if L.

IIIIIIIIIIIIIIIIIIIII I'll!!! n Inventor; Shepard Rcberts, by /Q% 4 HisAttorney.

United States Patent SHEAR TYPE PIEZO-ELECTRIC DEVICE Shepard Roberts,Scotia, N.Y., assignor to General Electric Company, a corporation of NewYork Application October 30, 1952, Serial No. 317,612 8 Claims. (Cl.179-10041) The; present invention relates to electromechanicaltransducers and. comprises both improved piezo-electric devices and animproved method of producing such devices.

It is known that various ceramic materials, and in particular thetitanates of some alkaline earth metals may be given piezo-electricproperties either by the sole action of a strong polarizing electricfield or by the conjoint action of mechanical deformation and thepolarizing electric field.

' Some applications of electromechanical translation devices requirethat the device shall respond piezo-electrically with a high ratio ofelectrical voltage to the applied mechanical deforming force. It hasbeen suggested that a plate or slab of piezo-sensitive ceramic materialshall be mounted between conductors applied to the opposite flatsurfaces of the plate and that piezo-electric properties should beimparted by applying to the plate a transverse force during polarizationby an electric field. V

. The voltage produced by deformation of such a polarized plate heldbetween oppositely disposed electrodes necessarily would be restrictedby the structural limitations of such a unit. The voltage in apiezo-electric unit depends on the electric field set up by stress inthe material. In the suggested device the distance between the oppositesides of a plate to which the electrodes are applied is the shortestdimension of the unit. The voltage of a unit having such configurationcannot be increased by merely increasing the thickness of the plate andthus increasing the distance between the electrodes as this would causethe force applied during deformation to be distributed in such a waythat the unit stress in the material would be disadvantageouslyscattered with a corresponding reduction of electric field. Hence, thevoltage produced by such unit necessarily must be low.

In accordance with my present invention, improved piezo-electrictransducers of higher voltage output have been provided by polarizingelongated units, and preferably units of cylindrical or tubularconfiguration, in an electric field while applying torsional or twistingstresses in the material in the direction of the major axis. A featureof my invention also comprises the spatial relation of the electrodes onthe elongated unit. The electrodes are applied at regions axially spacedapart and preferably the electrodes embrace circumferentially the endregions of cylindricalunits and preferably tubular units.

Fig. l of the accompanying drawing shows an elongated ceramic unit beingsubmitted to polarization at an elevated temperature while applyingtorsional forces; and Fig. 2 shows a tubular piezo-electric elementmounted on a tone arm of a phonograph.

' Referring to the drawing, an elongated ceramic unit 1 is shown asfixedly attached to a stationary base 2. The unit 1 may consist ofparticles of a titante of an alkaline earth metal. Particles of titanatemay be bonded to one another by sintering the particles at an elevatedtemperature;"- They maybe bonded with asuitable mineral 2,902,545Patented Sept. 1, 1959 'ice material, as for example finely divided TiOor with other suiable bonding material. Although my invention is notrestricted to any specific material or class of piezoelectric materials,a preferable material is barium titanate.

Electrodes 3, 4 mounted on the unit 1 of Fig. 1 (a tube or a rod) atregions spaced apart are connected to conductors 5, 6 which areconnected in turns to parts of the apparatus, the latter being groundedas indicated at 7. Such electrodes may consist of encircling bands ofsilver, copper or other metal, which may be applied as a paste of thechosen metal in a finely divided state. The central electrode 8 isconnected to a conductor 9 whereby during operation a polarizingpotential may be applied from a suitable source (not shown).

The piezo-electric unit 1 at its end opposite the fixed support 2, islinked to a holder 10 which in turn is connected to a rotatable shaft11. The holder 10 consists of several separable parts. The part 12 isscrewed, as indicated, to the shaft 11. A protuberant tip 13 thereofengages with a slot in the link 14 which in turn engages with a slot inthe part 15 which grasps the end of the ceramic unit 1. The end of theshaft 11 which is connected to the holder 1% is supported on a bearingwithin an orifice of the end wall 16 of a housing 17 surrounding theceramic unit 1. The base 2 is supported by the opposite end wall 16' ofthe housing 17. The outer extremity of the shaft 11 is supported in abearing 18 and has attached thereto a lever 19. At the end of the lever19 is afiixed a post 20 to which may be applied a weight 21 by a wire orcord 22. The weight of the lever 19 itself and the added weight 21attached thereto when the lever is in a horizontal position causes atwisting force to be applied to the ceramic unit 1 without any bendingforce as the unit to be polarized is supported at both ends. The lever19 is shown in an upright position, as in this position it is morereadily illustrated. However, it is assumed to be oriented horizontallywhen twisting force is applied to the unit 1.

During polarization, the unit 1 preferably is suitably heated, anexternal electric heater being indicated at 23. If the unit is heatedduring polarization to above the Curie temperature, which for bariumtitanate approximates C., the intensity of electric field which isrequired for polarizing the ceramic material need not be so high as inthe absence of heating. The polarizing field must be continued, however,until the temperature again has been reduced below the Curietemperature.

If a temperature above the Curie point is chosen, a polarizing field of2.5 kilovolts or higher per centimeter should be applied to the materialby connection of a unidirectional source of chosen potential to theconductor 9, the return path being the conductors 5, 6. The timerequired for the unit 1 to cool through the Curie point is sufiicient toachieve permanent piezo-electric effect. If polarization is carried outat room temperature, a stronger polarizing field is desirable. Theapplication of a weak electric field such as 2.5 kilovolts percentimeter should be continued for several hours; but if the appliedfield is as high as 25 kilovolts per centimeter, polarization will besubstantially instantaneous.

During the application of the electric field, a mechanical twistingforce is applied at the free end of the unit by the rotation of leverarm 19, which force produces a shear stress in the material. Theelectric field, which is parallel to the axis between the electrodes,induces a polarization which is modified by the mechani cal stress so asto be inclined at an angle to this axis. Neither the electric field northe twisting force is removed until the temperature again has beenreduced substantially below the Curie temperature or the polarizationhas been completed at room temperature. Upon release ;of the twistingforce and removal of the applied potential, the polarized unit ispiezo-electrically sensitive to torsion, making such a unit useful forvarious purposes. One field of use is the measurement of varying torquein machinery.

As illustrated in Fig. 2, a twist-polarized unit 1 (here being shown as.a tube) is attached to the tone arm of a phonograph which is providedat its free end with a stylus 26. The bearing 27 at the tip of thestylus engages with the sound groove of phonograph record 28. The stylus26 should extend essentially tangent to the grooves in the record to theradius of the phonograph plate 28.

As the record is revolved by the motor 29, a variable twisting force isapplied to the polarized piezo-electric unit 1 by the stylus 26 bearingagainst the variable sound track on the record 23. The resultingvoltages are impressed upon appropriate electric amplifying apparatus(not shown) whereby the recorded sound is reproduced. For this purpose,the end electrodes 3, 4 may be connected to ground by the conductors 3iand 32. The central electrode 8 is connected by a conductor 34 tosuitable sound amplifying and reproducing equipment (not shown).

Intermediate electrode 3 may be omitted and polarizat ion accomplishedby applying the designated potentials between electrodes 3 and 4 whileunit 1 is under the above described torsional stress. In this case thesound reproducing equipment is connected to receive the piezoelectricvoltage developed between electrodes 3 and 4, only one of which may begrounded. It will be appreciated, however, that the use of an additionalcentral electrode 8 as described above has the advantages of permittingthe grounding of both supporting structures 2. and 10 of Fig. l; ofreducing the polarizing potentials which need be applied to electrodes 3and 4 for the distance between them; and of producing oppositelydirectedpolarization in the portions of unit l on either side of the centralelectrode 8 with the result that a symmetrical push-pull piezo-electricvoltage is generated producing greater electrical charge betweenelectrode 8 and the grounded electrodes 3 and 4 when the polarized unit1 is subject to torsion.

The following results were obtained with a barium titanate tube whichwas 4 inches long, having an outside diameter of 5.2 millimeters, theinside diameter being one-half of this dimension. Three conductingsilver bands were applied, spaced one inch apart. The central band wasconnected to a D.-C. potential of about 6,000 volts, the outer bandsbeing grounded. A weight of 500 grams was applied to a handle (such as19 when horizontal) and having a length of 10 cm., to produce a twistingtorque. The tube was heated to 125 C., with the polarizing potentialbeing applied and then was cooled to 60 C., the polarizing voltage andtorque being continuously applied. At 60 C. the voltage and torque wereremoved. The tube finally was cooled to room temperature.

The tube so treated exhibits high piezo-electric voltages when twisted.The charge generated by applying to such a unit a twisting force set upby a 500 gram weight at the end of the rod 19 at a radius of 10 cm. was25,000 micro-microcoulombs.

This figure corresponds to a piezo-electric shear modulus about equal tothe best values obtained for transverse piezo-electric modulus in bariumtitanate polarized by methods heretofore employed. The improvedtransducer made as herein described takes advantage of the improvedgeometrical disposition of the electrodes and provides an improvedmethod of activation. One of the advantages is a higher voltage for agiven distorting force by reason of the greater distance between theelectrodes.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A twister type of electromechanical transducer com- 4- prising anelongated tube consisting mainly of polarized barium titanate having acomponent of polarization inclined at an angle to the longitudinal axisof said tube and a pair of signal electrodes encircling opposite ends ofthe external surface of said tube, said tube generating voltages betweensaid electrodes when subjected to twisting forces about a longitudinalaxis of said tube.

2. An electromechanical transducer comprising an elongated tube ofpolarized barium titanate having a component of polarization inclined atan angle to the longitudinal axis of said tube, a pair of signalelectrodes encircling and secured to longitudinally-spaced portions ofthe exterior surface of said tube, said tube having piezo-electricproperties producing a voltage at said electrodes in response tomechanical forces tending to twist said tube about its longitudinalaxis.

3. An electromechanical transducer comprising an elongated body ofpolarized piezo-electric ceramic material having a component ofpolarization inclined at an angle to the longitudinal axis of said bodyand a pair of signal electrodes mounted on the exterior surface of saidbody at regions longitudinally spaced apart, said body being activatedto have electromechanical properties in response to mechanical forcestending to twist said body about its longitudinal axis.

4. A transducer comprising an elongated tube of barium titanate, threeelectrodes encircling and secured to equally longitudinally-spacedexterior surface portions of sald tube, said tube havingoppositely-directed piezoelectric polarization having a componentthereof inclined at an angle to the longitudinal axis of said tube fromthe centrally located electrode to each of the endwise electrodes andcharacterized by generating piezo-electric voltages between said centralelectrode and each endwise electrode when the tube is subjected tomechanical forces tending to twist the tube about its longitudinal axis.

5. A phonographic pick-up device comprising the combination of anelongated tube of piezo-electric material polarized with a component ofpolarization inclined at an angle to the longitudinal axis of said tube,a pair of signal electrodes applied to the exterior surface of said tubeadjacent to opposite ends thereof, a third signal electrode contactingsaid surface upon a region intermediate said ends, and a stylus mountedupon said tube adjacent an end thereof so disposed that when said stylusengages a sound track said tube is subjected to a twisting moment, saidtube being activated to have piezo-electric properties in response tomechanical forces which tend to twist said body about the longitudinalaxis thereof when said stylus engages with a sound track.

6. A phonographic pick-up device suitable for the electromechanicalreproduction of recorded sound comprising the combination of anelongated tube of barium titanate, signal electrodes applied to thelateral surface of said tube adjacent the ends thereof and at anintermediate region, and having mounted thereon an outwardly projectingstylus tipped with a bearing for engaging with a sound track so disposedthat when said stylus engages said sound track said tube is subjected toa twisting moment, said tube being electrically polarized with acomponent of polarization inclined at an angle to the longitudinal axisthereof and thereby being responsive to twisting forces to generatevoltages at said electrodes.

7. An electromechanical transducer comprising an elongated body ofpiezoelectric sensitizable ceramic material and signal electrodesmounted on the same surface of said body at regions longitudinallyspaced apart, said body having piezoelectric properties with a componentof polarization inclined at an angle to the longitudinal axis of saidbody and generating voltages between said electrodes when subjected totwisting forces about the longitudinal axis of said body.

8. The electromechanical transducer of claim 7 wherein the elongatedbody of piezoelectric sensitizable ce- 5 6 ramic is in the form of atube, and the longitudinally 2,540,851 Wiggins et a1. Feb. 6, 1951spaced electrodes encircle the outer surface of the tube. 2,575,999Cordes Nov. 20, 1951 2,596,494 Lynch May 13, 1952 References Clted 1nthe file of thls patent 2,616,223 Jonker Nov- 4, 1952 UNITED STATESPATENTS 5 2,624,853 Page Jan. 6, 1953 1,861,862 Hund June 7, 19322,625,663 Howatt Jan. 13, 1953 2,486,560 Gray Nov. 1, 1949 2,702,427Roberts Feb. 22, 1955 2,518,861 Burtch Aug. 15, 1950 2,706,326 MasonApr. 19, 1955 2,540,187 Adler Feb. 6, 1951 2,742,614 Mason Apr. 17, 19562,540,412 Cherry Feb. 6, 1951 10 2,756,353 Samsel July 24, 1956

